| The formation of microporosity during solidification of aluminum alloy castings will greatly reduce their machnical properties, and thus limit the applications of aluminum casting products in critical safety components. This thesis is focused on the numerical modeling of microporosity formation caused by hydrogen evolution in binary and ternary Al alloys during solidification. The main results are summarized as follows.The previously established two-dimensional (2D) cellular automaton (CA)-finite difference method (FDM) model for the simulation of dendrite growth coupled with microporosity formation of Al-Si alloys is modified. In the modifed model, the approach oflocal solute equilibrium at the solid/liquid (SL) interface is adopted to calculate the growth kinetics of the primary a-Al dendrites.. A CA model is developed to simulate the evolution of irregular eutectic microstructures in solidification of Al-Si alloys. The newly developed eutectic growth model is coupled with the modified dendrite and microporosity model to develop a multi-phase cellular automaton (MCA)-finite difference method (FDM) model for the simulation of dendrite, eutectic growth coupled with microporosity formation in solidification of binary Al-Si alloy system. The MCA-FDM model is applied to investigate the effect of modification treatment on the morphology of eutectic β-Si phase, and the effects of initial hydrogen content and cooling rate on the microporosity formation. The results show that the MCA-FDM model can effectively simulate the realistic microstructure and microporosity evolution during solidification of Al-7wt.%Si alloy. The thick flake or needle eutectic β-Si becomes tiny short rod or paticle structure through modification treatment. The competitive growth between pores with different sizes is observed. The pores with larger size are able to grow preferentially, while the growth of the small pores is limited. With the occurrence of eutectic reaction, the nucleation and growth of microporosity are facilitated, leading to the increase of volume fraction and density of microporosity. With the increase of initial hydrogen content, pore volume fraction, average pore radius and maximum pore radius increase. As cooling rate increases, dendrites become finer, leading to the the decrease of pore volume fraction, average pore radius and maximum pore radius. Moreover, the pore size distribution becomes more uniform, but the density of porosity increases. A MCA-FDM-PanEngine model is developed for the simulation of dendrite, eutectic and microporosity formation in solidification of ternary Al-Si-Mg alloy by means of coupling the eutectic growth model with the previously proposed 2D dendritic and micropore growth model of ternary alloys. The model is applied to simulate the evolution of equiaxed dendrites with eutectic growth in solidification of ternary Al-Si-Mg alloys.The microporosity formation coupled with equiaxed orcolumnar dendrites, and eutectic growth in solidification of Al-Si-Mg alloys are also simulated by the MCA-FDM-PanEngine model. The effects of cooling rate and modification treatment on the formation of microporosity are investigated. The results show that the MCA-FDM-PanEngine model can effectively simulate the realistic equiaxed dendrites, columnar dendrites and microporosity evolution in solidification of Al-rich ternary alloy under different solidification conditions. The morphology of microporosity is influenced by the growth of solid phase and becomes irregular. A small amount of Mg2Si precipitates out at the end of solidification process of Al-5wt.%Si-0.9wt.%Mg and Al-7wt.%Si-0.4wt.%Mg alloys. The effect of cooling rate on microporosity formation is consistent with the simulated results of binary alloys. The surface tension of aluminum alloy melt can be reduced by modification treatment, which leads to the increase of pore volume fraction, average pore radius and maximum pore radius. The trends of simulation results agree reasonably well with the experimental data and simulation results reported in literature.A microstructure simulation software composed of the calculation and visualization parts is developed. The kernel algorithm of the calculation part developed by Qt is based on the MCA-FDM model, and the visualization part developed by MFC can be aplied to display the simulation results graphically. The microstructure simulation software has a friendly user interface with simple and convenient operations. Calculation part contains three basic modules:dendrite moudle, eutectic moudle and microporosity moudle. These three modules can be applied to simulate dendrite growth, eutectic growth and dendrite, eutectic growth coupled with microporosity formation in solidification of binary alloys, respectively. While the visualization part can be applied to graphically output the simulated microstructure morphology during solidification of binary alloys... |
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